For image display apparatuses such as an HMD (Head Mount Display) which is mounted on a head of a user in order to display an image, conventionally, there have been proposed various schemes such as a scheme of employing a pixel type display device such as a liquid crystal element or an organic EL to be used as an image display section, and a scheme of scanning a laser beam in a two-dimensional manner to directly depict an image on a retina of an eye.
Such image display apparatus needs to be small in size and light in weight so that a user can use the image display apparatus for a long period of time while reducing a burden to be imposed on the user when the image display apparatus is mounted on the user. Furthermore, such image display apparatus is designed like a typical eyewear, so that a user can do anything while constantly putting on this image display apparatus just like a typical eyewear.
According to the scheme of employing the pixel type display device, however, an eyepiece optical system including a display section, a prism that guides light emitted from the display section to an eye, and a half mirror is increased in size in order to realize higher image quality and a wider angle of view. Consequently, it is difficult to realize size reduction and weight reduction with regard to the image display apparatus.
Moreover, the large eyepiece optical system described above is mounted on the user so as to cover his/her eyes, and therefore has a shape like a goggle or a helmet rather than an eyewear. Consequently, it is not possible to attain a feeling of natural mounting, and therefore it is difficult to realize an image display apparatus like a typical eyewear.
On the other hand, a retina scanning image display apparatus that employs a laser scan scheme includes a small MEMS (Micro-Electro-Mechanical-System) mirror device. This structure brings about an advantage in that the image display apparatus can be significantly reduced in size.
Further, there has also been proposed a different retina scanning image display apparatus that employs the laser scan scheme. This image display apparatus includes a hologram mirror in place of a prism and a half mirror. According to this structure, an eyepiece optical system is reduced in size, so that this image display apparatus can be formed in an eyewear type (see, for example, Patent Document 1).
FIGS. 8A through 8C each illustrate one example of a structure of the image display apparatus that employs the laser scan scheme. Specifically, FIG. 8A shows a plan view of the image display apparatus that employs the laser scan scheme. FIG. 8B shows a side view of the image display apparatus that employs the laser scan scheme. FIG. 8C shows the image display apparatus that employs the laser scan scheme when the image display apparatus is seen from an eye side.
FIGS. 8A through 8C each illustrate only a right side with regard to a head portion of a user and the scanning image display apparatus. In a case of a both-eyes type, this apparatus has a laterally symmetric structure.
The scanning image display apparatus shown in FIGS. 8A through 8C has the following structure. That is, a temple 11 is equipped with a light source section 1 that emits a laser beam 2, a scan mirror 3 that scans the laser beam 2 in a two dimensional manner, and a control section 14 that controls these members.
The scanning image display apparatus also includes an eyewear lens 12, and a hologram mirror 13 that is formed on a surface of the eyewear lens 12. The laser beam 2 is projected by the scan mirror 3 onto the eyewear lens 12, is reflected by the hologram mirror 13, and then enters an eye 17 of the user. Thus, an image is formed on a retina of the eye 17. For example, the hologram mirror 13 includes a photopolymer layer having a Lippmann volume hologram formed thereon. The hologram mirror 13 has such a wavelength selectivity to reflect only a wavelength of a laser beam. As a result, the user can visually recognize both outside scenery and an image depicted by the laser beam at the same time.
According to the foregoing conventional structure, the hologram mirror 13 is irradiated with the laser beam 2 from the scan mirror 3 without such a situation that the laser beam 2 is shielded by a user's face. Thus, the laser beam 2 is obliquely projected onto the eyewear lens 12 at an incident angle α. Consequently, there arises a problem that an image to be projected onto the hologram mirror 13 becomes distorted in a trapezoid shape, like an oblique projection region 8 shown in FIG. 8C.
Typically, when a rectangular image is projected from an oblique position onto a projection plane, a scan beam is expanded at a side distant from a scan center, so that the resultant projection region has a trapezoid shape in which a side close to the scan center is narrow whereas a side distant from the scan center is wide. For this reason, when light reflected by the hologram mirror 13 enters the eye 17 to reach the retina, an image to be recognized by the user becomes distorted in a trapezoid shape.
Normally, a front projector or the like performs image processing on an image to correct such a trapezoid distortion. Herein, a rectangular display region is determined based on a length of shorter one of an up side and a bottom side in a trapezoid shape, and an image is not displayed on a portion protruding from the rectangular region (hereinafter, referred to as an invalid scan region). Thus, a user can recognize a rectangular image like a display region 9 (see FIG. 8C).
According to this method, however, the image is displayed in a downsized manner at a side where the projection region becomes widened, so that a displayable resolution is lowered. Moreover, as the invalid region is larger, a period of time that an image can be displayed within one frame becomes short, and the image becomes darkened. In order to maintain the brightness of the image, an output from the light source needs to be raised, which results in an increase in the power consumption.
In order to solve the foregoing problem, there has been proposed a method of controlling a drive amplitude and a drive speed of a scan mirror to correct a shape of a scan region and a scan line pitch (see, for example, Patent Document 2).
In this method, an MEMS mirror quickly performs a scan operation in a first direction (a fast scan direction) whereas a vertical deflector (a so-called galvanometer mirror) which can be driven at a free waveform slowly performs a scan operation in a second direction (a slow scan direction). Then, a deflection amplitude of the MEMS mirror is changed. This change offsets widening of a scan width in a case of oblique projection. Further, this change allows control of a deflection speed of the galvanometer mirror, and also allows correction of widening of the scan line pitch.
However, the described conventional structure also has the following problems.
Herein, the deflection amplitude of the MEMS mirror is changed in accordance with the scan operation in the slow scan direction. Therefore, a drive voltage on a side where the amplitude is suppressed is lowered or a distance between a coil and a permanent magnet in a MEMS mirror drive section is increased. As a result, a drive force for the MEMS mirror is weakened and the deflection amplitude is decreased. In the MEMS mirror which is vibrated while being resonated, particularly, the vibration is maintained at a certain level by inertial motion, so that the change in the amplitude does not necessarily follow a change in a drive signal.
Further, the slow scan operation is performed only in one direction. This structure makes it more difficult that a change in the amplitude of the MEMS mirror follows a fast vertical feedback.
For the correction of the scan line pitch, the galvanometer mirror which can be driven at a free waveform is used for performing the slow scan operation. In order to drive the galvanometer mirror, an actuator needs to be provided to generate a satisfactory drive force in an entire angular displacement. This actuator hinders the size reduction of the image display apparatus.
On the other hand, the resonance mirror can achieve a large displacement with a small drive force. Therefore, use of the resonance mirror is suitable for the size reduction of the image display apparatus. However, a vibration waveform is in a sine wave shape, so that the resonance mirror cannot be driven at a free waveform.
In the eyewear type HMD described above, that is, in an image display apparatus that requires a very small structure, a biaxial resonant MEMS mirror is suitably employed because this MEMS mirror can perform a biaxial scan operation in one chip. Because of the reasons described above, however, the MEMS mirror cannot be driven so as to correct the shape of the scan region and the scan line pitch.